The present invention is directed to the preparation of composite printing plates and, more particularly, to the direct transfer of digital images to such composites without the use of phototools or photomasks.
Relief image printing plates are used in both flexographic and letterpress processes for printing on a variety of substrates, including paper, corrugated stock, film, foil, and laminates. The photocurable elements that are used to make relief plates typically include a support layer and one or more layers of photocurable polymer in the form of solid sheets. The printer typically peels a cover sheet from the element to expose the photocurable polymer and places a silver halide photographic negative or some other masking device upon the photopolymer. The negative-bearing photocurable element then is exposed to ultraviolet (UV) light through the negative, thereby causing exposed areas of the element to harden, or cure. After the uncured areas of the element are removed, cured polymer remains as the relief printing surface.
Corrugated boxes and other, relatively large objects that are printed using relief image printing plates often bear actual printing on only a small portion of their total surface area. One way to print such an object is to prepare a single relief image plate having a surface area corresponding to the total surface area of the object. Since only a portion of the object""s surface needs to be printed, however, only a portion of the relief image plate will actually used for ink transfer. The remainder of the plate will be unused and, essentially, wasted.
To minimize such waste, those skilled in the art often print relatively large objects with composite printing plates that are prepared by mounting a plurality of relief image printing plates on a common carrier sheet. The individual plates, however, are mounted only on those portions of the carrier that correspond to the portions of the object that actually need to be printed. Although such composite plates do minimize waste, the current system for mounting their constituent relief image plates is laborious and requires careful adhesion of the plates to the carrier while assuring registration to within 0.005 inches on-press for high quality printing and multi-color reproduction. For multi-color reproduction, wherein a single plate is used for printing each of the individual colors, accurate registration of the plates with respect to one another is crucial.
Consequently, there remains a need in the art for alternative processes for preparing composite printing plates. In particular, there remains a need for alternative processes for accurate registration of the constituent relief image printing plates or for processes wherein accurate registration is not necessary.
It is one object of the present invention to provide methods for preparing composite printing plates.
It is another object of the invention to provide methods for registration of at least one relief image printing plate on a common carrier sheet.
It is a further object to provide methods for printing registration information directly onto the surface of the carrier sheet using a computer.
It is yet another object to provide methods for transferring an electronically stored negative image directly onto a composite printing plate.
These and other objects are satisfied by the present invention, which provides methods for preparing high-quality composite printing plates without the need for individual registration of constituent relief image plates. These methods comprise the steps of disposing a photocurable element upon a surface of a substantially planar carrier sheet in approximate register and then transferring a computer-generated negative onto element. In preferred embodiments, the methods of the invention comprise the steps of providing at least two substantially planar photocurable elements that have first and second opposing major faces of defined surface area, disposing a first face of the photocurable elements upon a first face of a substantially planar carrier sheet having first and second opposing major faces of defined surface area, and ejecting the negative-forming ink from an ink jet print head onto second faces of said photocurable elements.
Approximate registration of the photocurable elements can be achieved by transferring computer-generated registration information to a surface of the carrier sheet. This can be achieved, for example, by transferring some visually perceptible material (such as ink from an ink jet print head) onto the sheet, or by scoring or otherwise deforming the sheet. The registration information can, for example, comprise a series of images whose respective shapes correspond to the outlines of the individual photocurable plates. Following the transfer of registration information, photocurable elements are placed on the carrier sheet in accordance with the positions dictated by the registration information.
Transfer of the computer-generated negative to composite plates of the invention preferably is achieved by ejecting a negative-forming ink from an ink jet print head. The ink preferably is substantially opaque to actinic radiation in at least one wavelength region effective to cure photocurable material within the element and substantially resistant to polymerization upon exposure to actinic radiation in the wavelength region. Following the negative transfer step, the ink-bearing plate can be exposed to actinic radiation in the wavelength region for a time and under conditions effective to cure exposed areas of the photocurable material, and unexposed (i.e., uncured) areas then are removed to provide the relief printing surface.
The present invention further provides negative-bearing composite printing plates produced in accordance with the foregoing methods. In certain embodiments, the plates comprise a plurality of photocurable elements disposed on a substantially planar carrier sheet, at least two of the photocurable elements including a support layer, photocurable material disposed upon the support layer, and negative-forming ink disposed upon at least a portion of the surface of the photocurable material.